1. Field of the Invention
The invention relates to a process for producing a planar body of an oxide single crystal.
2. Description of the Related Art
A single crystal of lithium potassium niobate and a single crystal of lithium potassium niobate-lithium potassium tantalate solid solution have been noted especially as single crystals for blue light second harmonic generation (SHG) device for a semiconductor laser. The device can emit even the ultraviolet lights having the wavelengths of 390 nm or so, thus the crystals can be suitable for wide applications such as optical disk memory, medicine and photochemical fields, and various optical measurements by using such short-wavelength lights. Since the above single crystals have a large electro-optic effect, they can be also applied to optical memory devices using their photo-refractive effect.
However, for an application of a second harmonic generation device, for example, even a small fluctuation in a composition of the single crystal may affect the wavelength of the second harmonic wave generated by the device. Therefore, the specification of the range of the composition required for said single crystals is severe, and the fluctuation in the composition should be suppressed in a narrow range. However, since the composition consists of as many as three of four components, growing a single crystal at a high rate is generally extremely difficult to achieve, while controlling the proportion of the components to be constant.
In addition, for optical applications, especially for an application for the second harmonic wave generation, a laser beam having a short wavelength of, for example, about 400 nm needs to propagate in the single crystal at as a high power density as possible. Moreover, the photo deterioration has to be controlled to the minimum at the same time. In this way, since controlling the photo deterioration is essential, the single crystal has to possess a good crystallinity for this purpose.
Moreover, lithium niobate and lithium potassium niobate can be substituted between cations, thus solid solution in which the cations are solid-solved is produced. Therefore, controlling the composition of the melt needs to grow a single crystal of a specific composition. From such a background, a double crucible method and a method of growing a crystal while feeding raw materials have been examined mainly for the CZ method and the TSSG method. For example, Kitamura et al. tried to grow a lithium niobate single crystal of a stoichiometric composition by combining an automatic powder feeder to a double crucible CZ method (J. Crystal Growth, 116 (1992), p.327). However, it was difficult to enhance a crystal growth rate with these methods.
NGK Insulators, Ltd. suggested a micro pulling-down method for growing the above single crystal with a constant compositional proportions, for example, in JP-A-8-319191. In this method, a raw material, for example, comprising lithium potassium niobate is put into a platinum crucible and melted, and then the melt is pulled down gradually and continuously through a nozzle attached to the bottom of the crucible. The micro pulling-down method can grow a single crystal more rapidly than the CZ method or the TSSG method does. Moreover, the compositions of the melt and the grown single crystal can be controlled by growing the single crystal continuously with feeding the raw materials for growing the single crystal to the raw material melting crucible.
However, there is still a limitation in using a micro pulling-down method to grow a good single crystal plate (a planar body of a single crystal) continuously at a high rate. Because, when the planar body of the single crystal is pulled down with the seed crystal plate, cracks tend to occur near an interface boundary between the seed crystal and the planar body.
It is an object of the invention to prevent cracks occurring near the interface boundary between a seed crystal and a planar body and to grow a planar body of an oxide single crystal having a good crystallinity continuously and stably, when the planar body of the oxide single crystal is grown with the micro pulling-down method.
The inventors had examined various methods to grow planar bodies of oxide single crystals, which use the micro pulling-down method. As a result, the inventors found that cracks were likely to occur when a difference in lattice constant between the seed crystal and the planar body was large. Thus, the planar body having a good crystallinity may be continuously made by contacting a planar seed crystal to a melt, pulling down the melt from an opening of a crucible by lowering the seed crystal, forming a planar body following the seed crystal, and controlling difference in lattice constant between each of crystal axes of the seed crystal and corresponding crystal axes of the planar body at 0.1% or less, respectively.
In this case, the lattice constant of each crystal axis of the planar body can be adjusted by controlling the proportions of the respective components in the crucible. Taking lithium potassium niobate, for example, the lattice constant of each crystal axis in the grown planar body can be changed by slightly changing a relative ratio of niobium, lithium and potassium in the crucible.
Preferably, the differences between the lattice constants of the planar body and the respective ones of the planar seed crystal are further reduced as the width of the planar body become large. Specifically, when the width of the planar body is 30-50 mm, it is more preferable to control the differences between the corresponding lattice constants at 0.06% or less. When the width of the planar body is 50 mm or more, it is more preferable to control the differences of the lattice constants at 0.04% or less.
FIG. 1 is a schematic sectional view of a manufacturing apparatus for growing a single crystal. FIGS. 2(a) and (b) represent steps of pulling down a planar body of the single crystal.
A crucible 7 is placed in a furnace body. An upper furnace unit is arranged to surround the crucible 7 and an upper space 5 thereof, and has a heater 2 buried therein. A nozzle 13 extends downwardly from a bottom part of the crucible 7. The nozzle 13 comprises a connecting-tube portion 13a and a planar expanded portion 13b at the lower end of the connecting-tube portion 13a. In FIG. 1, only a cross sectional view of the planar expanded portion 13b is shown. The connecting-tube portion 13a and the planar expanded portion 13b can be changed variously in shape. Both 13a and 13b can also be arbitrarily changed in combination. A slender opening 13c is formed at the lower end of the planar expanded portion 13b, and a vicinity of the opening 13c is a single crystal-growing portion 19. A lower furnace unit 3 is arranged to surround the nozzle 13 and a surrounding space 6 thereof, and has a heater 4 buried therein. The crucible 7 and the nozzle 13 are both formed from a corrosion-resistant conductive material.
One electrode of a power source 10 is connected to a point A of the crucible 7 with an electric cable 9, and the other electrode of the power source 10 is connected to a lower bent B of the crucible 7. One electrode of an another power source 10 is connected to a point C of the connecting-tube portion 13a with an electric cable 9, and the other electrode of the power source 10 is connected to a lower end D of the planar expanded portion 13b. These current-carrying systems are isolated from each other and configured to control their voltages independently.
An after-heater 12 is located in the space 6 to surround the nozzle 13 with a distance. An intake tube 11 extends upwardly in the crucible 7 and an intake opening 22 is provided at the upper end of the intake tube 11. The intake opening 22 slightly protrudes from a bottom portion of a melt 8.
The upper furnace unit 1, the lower furnace unit 3 and the after-heater 12 are allowed to heat for setting an appropriate temperature distribution in each of the space 5 and space 6. Then a raw material for the melt is supplied into the crucible 7 and the electricity is supplied to the crucible 7 and the nozzle 13 for heating. In this condition, the melt slightly protrudes from the opening 13c at the single crystal-growing portion 19.
In this condition, a planar seed crystal 15 is held with a holder 21 at both side faces and moved upwardly as shown in FIG. 2(a), and the upper surface of the seed crystal 15 is contacted with the melt protruding from the opening 13c. At that time, a uniform solid phase-liquid phase interface (meniscus) is formed between the upper end of the seed crystal 15 and the melt 18 pulled downwardly from the nozzle 13. Then, the seed crystal 15 is lowered as shown in FIG. 2(b). As a result, a planar body 14 is continuously formed on an upper side of the seed crystal 15 and pulled downwardly.
The lattice constant is measured by an X-ray diffraction apparatus (MRD diffractometer, manufactured by Philips). If an unequivalent crystal axis exists in an oxide single crystal, differences in lattice constants between each crystal axis of the seed crystal and each corresponding axis of the planar body, respectively, have to be 0.1% or less.
An oxide single crystal is not particularly limited, but, for example, lithium potassium niobate (KLN), lithium potassium niobate-lithium potassium tantalate solid solution (KLTN: [K3Li2xe2x88x92x(TayNb1xe2x88x92y)5+xO15+2x]) lithium niobate, lithium tantalate, litium niobate-lithium tantalate solid solution, Ba1xe2x88x92xSrxNb2O6, Mn-Zn ferrite, yttrium aluminum garnet substituted with Nd, Er and/or Yb, YAG, and YVO4 substituted with Nd, Er, and/or Yb can be exemplified.